An apparatus of this type is known in which a changeover valve is operated in one direction or the other by turning of a steering shaft, so that a supply oil passage connected to an operational oil pressure source, such as an oil pressure pump or the like, may be selectively connected to the left or right chamber of a power cylinder, at least one oil pressure reaction chamber being arranged externally of the changeover valve so as to be reduced in volume by the operation in either direction of the changeover valve to thereby oppose the same, the oil pressure reaction chamber being connected via a control valve to the foregoing supply oil passage, the supply oil passage being provided with a divergent exhaust oil passage connected thereto through a throttle and having at its downstream portion a control member serving to control the flow rate according to vehicle speed, the control valve being constructed so as to move towards its opening side by a differential pressure existing between the supply oil passage and the exhaust oil passage. Thus the oil pressure generated in the supply oil passage in proportion to road surface resistance may be applied to the oil pressure reaction chamber, while being adjusted according to vehicle speed by the action of the control valve.
This conventional construction is advantageous in that, during low speed travel of the vehicle, the steering force required of the driver can be kept at a constant and small value regardless of the change of the road surface resistance, whereby a steering operation can be carried out with a relatively light force, whereas during high speed travel of the vehicle, the steering force can be increased in proportion to the road surface resistance so that oversteering can be effectively prevented. This conventional construction, however, is deficient in that, when the vehicle is driven at a low speed or is at a stop, the foregoing control member is brought into such a condition that the exhaust passage is nearly closed thereby, so that the high oil pressure within the supply oil passage acts on the control member, without being reduced in pressure by the action of the throttle, and thus the control member is subjected to a high load and the durability or life span of the control member is lowered. Furthermore, the control member is often damaged by the high load, and as a result an accurate control according to the vehicle speed cannot be effected thereby.